Couldn't tell, but it looked loose on the side, so just deepen it, if that is the case, and you have the room to move. Are you using a tube miter program? They are free, and should make getting a fit pretty automatic.

you can see he's got the tube wrapped up in paper that is the output from a program.

The miter isn't quite there yet. Right at the ears, there would be a very large gap on the inside of the tube. A perfect miter will have very thin sections at the outside, you want to avoid that. You want the tubes to be touching each other on the ID of the mitered tube.

I saw Bernie Mikkelson at a frame builders show a few years ago displaying some bad lug work. There was a joint he pulled apart from an old Sears frame, where there was no miter at all. The ends of the tube had been bashed in with a hammer, shoved into a lug and brazed together. And the joint held up.

I have an old Raleigh Record frame lying around, and if you look into the BB shell you can see none of the tubes were mitered at all. Stays, ST, DT-- all straight cuts. The ST and DT were just jammed together in the shell, no cuts.

I'm not suggesting you start mitering with a hammer, but if the miters on your first frame turn out to be slightly off, don't lose sleep over it. Assuming you braze them well, you'll be fine, and you can work to improve the miters on future frames.

Back in the day of pressed steel lugs, I saw a bike made by one of the big name U.K. builders that had been cut apart as a display. The top tube was clocked like the Huffy in your link. I laughed and laughed. The sad thing is that the bike would have worked fine for anyone that it fit. For some reason they were trying to hype this builder like some other builders had been hyped. I don't think it worked, but not because of that display.